Switch the initialization of Objective-C message parameters (as occurs

Switch the initialization of Objective-C message parameters (as occurs
during message sends) over to the new initialization code and away
from the C-only CheckSingleAssignmentConstraints. The enables the use
of C++ types in method parameters and message arguments, as well as
unifying more initialiation code overall.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102035 91177308-0d34-0410-b5e6-96231b3b80d8